Journal of Geosciences, 56 (2011), 317–325

DOI: 10.3190/jgeosci.101

Original paper

Uranium–niobium-rich alteration products after “písekite”,
an intimate mixture of Y, REE, Nb, Ta, Ti-oxide minerals fromthe Obrázek I pegmatite, Písek, Czech Republic
Radek Škoda1*, Milan Novák1, Jaroslav Cícha2
Department of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic; rskoda@sci.muni.cz
Prácheň Museum, Velké náměstí 114, 397 24 Písek, Czech Republic*
Corresponding author
1
2

An improperly described association, further called “písekite” aggregate, of fine–grained, mostly metamict U, Th-enriched
Y, REE, Nb, Ta, Ti-oxide minerals from Obrázek I pegmatite, Písek, forms an intimate mixture of polycrase-(Y),samarskite-(Y) and fergusonite-(Y). It underwent a low-temperature hydrothermal alteration the products of which are
depleted in Y and REE > U and significantly enriched in Si, As and P. Two distinct assemblages of secondary phases
were recognized to be associated with “písekite” aggregate, namely (i) that developed within the central parts of theaggregate and (ii) pocket assemblage in albite adjacent to it. Both assemblages are dominated by Fe-rich smectite.
“Písekite” aggregate is intensively replaced by U, Nb, Ti, Fe, As, P-rich phases of variable chemical composition: phase
A (Nb > Ti > Fe > U > As) from the first assemblage, as well as phases B (U ~ Nb >Ti > As > P > Fe) and C (U > Nb >As > Fe > Ti) from the second assemblage. Associated pharmacosiderite and kahlerite/metakahlerite as well as Fe-richsmectite indicate a low temperature of the alteration processes (less than 150–200 °C). Whereas the “písekite” aggregate
was the likely source of U, Nb and Ti, arsenopyrite of the host pegmatite yielded As with Fe. The textural features showthat the Fe-rich smectite replacement by the U, Nb-rich phases A, B and C took place on a micrometer scale, and thesehighly heterogeneous U, Nb-rich phases likely represent a mixture of nanoscale, possibly amorphous hydrated oxides,
phosphates–arsenates as well as silicates of U, Fe and Ca.
Keywords: uranium, polycrase-(Y), samarskite-(Y), fergusonite-(Y), kahlerite, smectite
Received: 4 May 2011; accepted: 21 September 2011; handling editor: J. Plášil

1. Introduction
Primary Y, REE, Nb, Ta, Ti-oxide minerals from granitic
pegmatites enriched in U and Th (of the aeschynite,
euxenite, samarskite, fergusonite and pyrochlore groups)
commonly undergo intense hydrothermal alterations enhanced also by their tendency to become metamict withtime (Ewing 1975). Metamictization, i.e. the transition
from a crystalline to an amorphous state, is accompanied
by major changes in chemical and physical properties.
Amorphous minerals are usually hydrated and amorphization generally decreases their chemical durability.
Altered metamict Y, REE, Nb, Ta, Ti-oxide minerals
commonly show enrichment in Si, Al and Ca, whereas
Y and REE, in particular, are released from the system(e.g. Nasraoui and Bilal 2000; Ercit 2005; Geisler et al.
2005; Ruschel et al. 2010). Uranium and thorium arealso selectively removed, leaving behind B-site cations– Nb, Ta and Ti (Ewing 1975; Geisler et al. 2005). These
alteration products also occur in the metamict state andthey are generally assigned to the pyrochlore-groupminerals based solely on the chemical composition (e.g.,
Ercit 2005).

“Písekite” (Krejčí 1923) is an improperly describedfine-grained mixture of metamict Y, REE, Nb, Ta,
Ti-oxide minerals close to samarskite (Bouška and Johan 1972) from granitic pegmatites of the Písek region.
It is typically strongly altered and rare relics of primaryphases revealed that the “písekite” represents an intimate
mixture of several primary Y, REE, Nb, Ta, Ti-oxideminerals compositionally related to polycrase-(Y), samarskite-(Y) and fergusonite-(Y) (Škoda et al. 2010). They
are commonly intimately intergrown with columbite-(Fe)and zircon (Škoda et al. 2010). The list of secondary
minerals replacing “písekite” includes scheelite, ixiolite,
rutile, zirconolite, galena and REE,U-rich pyrochloregroup minerals (Breiter et al. 2010; Škoda et al. 2010).
In the studied assemblages, secondary minerals either
directly pseudomorph altered “písekite” aggregate or
they are developed in small pockets in primary albite
adjacent to the “písekite” aggregate. Moreover, these assemblages contain several phases hitherto unknown fromthe “písekite” aggregates. Textural relations and chemicalcomposition of the individual secondary phases are givenin the current text along with a discussion regarding their
possible source and mobility of the individual elements.
www.jgeosci.org

Radek Škoda, Milan Novák, Jaroslav Cícha

2. Methods
Electron microprobe analyses (EMPA) of carbon-coatedpolished sections were obtained by a Cameca SX 100
instrument (Joint Laboratory of Electron Microscopy andMicroanalysis, Institute of Geological Sciences, Masaryk
University, and Czech Geological Survey, Brno) using
the wavelength-dispersion mode. The following analytical conditions were applied: accelerating voltage of
15 kV, beam diameter of 2–5 μm, beam current of 20 nA,
counting times of 20 s for Nb, Ta, Ti, Ca, Y and 30–60
s for the other elements. The standards and lines used
included: Ti (Kα) – TiO; Nb (Lα), Fe (Kα) – columbite(Fe) (Ivigtut); Ta (Mα) – CrTa2O6; Mn (Kα) – Mn2SiO4; Ca(Kα), W (Mβ) – CaWO4; Na (Kα) – albite (Amelia mine);
P (Kα) – Ca5(PO4)3F; K, Al, Si (Kα) – sanidine (Eifel);
Mg (Kα) – Mg2SiO4; As (Lα) – lammerite (Guanaco), Y(Lα) – YAG; Sn (Lα) – SnO2; U (Mβ) – U; Th (Mα) – ThO2;
Sc (Kα) – ScVO4; Pb (Mα) – vanadinite (Mibladen); Zr(Lα) – ZrSiO4; La, Ce, Er, Yb (Lα); Pr, Nd, Sm, Gd, Dy,(Lβ) – La–Dy orthophosphates; Er – Y0.5Er0.5AlO3; Yb– YbAlO3. Europium, Tb, Ho, Tm and Lu were not measured. Data were reduced using the PAP routine (Pouchou
and Pichoir 1985). Relative errors are estimated to be <1 % at the >10 wt. % level, 10–20 % at the ~ 1 wt. % leveland >20 % at the <0.5 wt. % level. The smectite/chlorite
analyses were collected using 15 kV of accelerating voltage, 10 nA of beam current and 5 µm of beam size. The
kahlerite/metakahlerite was analyzed at 15 kV and 2 nA
by beam 5 µm in diameter. The standards were similar
to those described above.
The X-ray maps were acquired in continuous stagemove mode using WDS spectrometers, 15 kV of accelerating voltage, 60 nA of beam current, >1 µm beam size,
0.5 µm step and dwell time of 100 ms per pixel.

3. Occurrence
Variscan pegmatites from the Písek region cut migmatizedgneisses and amphibole–biotite syenites (durbachites) of
the Mehelník Massif, Moldanubian Zone (Novák et al.
1998; Breiter et al. 2010). Beryl pegmatites vary fromless evolved and small dikes with common tourmaline
and rare beryl to large, textually highly differentiated andgeochemically more fractionated pegmatite dikes, up to
25 m thick, with common tourmaline, beryl, and numerous accessory minerals (e.g., fluorapatite, niobian rutile,ilmenite, columbite–tantalite, “písekite”, monazite-(Ce),xenotime-(Y), zircon, cordierite–sekaninaite, almandine–
spessartine, pyrite, and arsenopyrite). Zoned pegmatites
typically show complexly zoned internal structure with aborder granitic unit (Kfs + Qtz + Pl + Bt), a graphic unit
(Kfs + Qtz ± Bt), blocky K-feldspar and large quartz core.
318

In highly evolved dikes common albite unit (Ab + Qtz ±tourmaline, rare muscovite) is developed between quartz
core and blocky K-feldspar and it contains majority ofaccessory minerals. The latest primary unit is represented
by thin aplitic veins (1–5 cm thick) cutting the remaining
textural/paragenetic units (Čech 1985). Beryl pegmatites
from the Písek area belong to beryl–columbite subtype ofthe rare-element class (Černý and Ercit 2005).
The pegmatite from Obrázek I quarry (GPS49°18′36.541″N, 14°11′40.068″E) is one the most
evolved and texturally differentiated pegmatite dikes in
the Písek region and involves all textural/paragenetic
units given above. Minerals from this locality were examined in several papers (e.g., “písekite” – Krejčí 1923,
Bouška and Johan 1972; tourmaline – Povondra 1981;
beryl – Sejkora et al. 1998; niobian rutile – Černý et al.
2007). However, internal structure and petrography of
the individual units as well as crystal chemistry mostof the minerals remain poorly known (Čech 1985; Breiteret al. 2010).

4. Results

4.1. “Písekite” aggregate and its alteration
products“Písekite” was found at several localities of more fractionated pegmatites in the Písek area. It forms elongatedgrains commonly arranged to radial aggregates, up to
2 cm long (Fig. 1). The color of “písekite” aggregatesvaries from brownish–black through greenish to yellow,
depending on the degree and style of hydrothermal alteration. Its aggregates are enclosed in albite, K-feldspar,
tourmaline and, rather rarely, in beryl. Relics of primarypolycrase-(Y), samarskite-(Y) and fergusonite-(Y), commonly ~10–500 μm in size, were sporadically found in
the altered mass (Fig. 2).
The polycrase-(Y) and samarskite-(Y) volumetricallydominate over the fergusonite-(Y) relics in the “písekite”
aggregates examined in this study. The alteration of aggregate propagates from rim inwards and along fractures;
effects of progressive alteration were observed in BSE
images (Fig. 2). Chemical compositions of the polycrase(Y) and samarskite-(Y) relics also show slight deviationfrom the ideal stoichiometry whereas chemical compositions of altered and the least altered parts differ significantly (Tab. 1). The altered areas are slightly enrichedin Si and strongly depleted in Y + REE relative to theirprecursors. The enrichment in Si (up to 7.1 wt. % SiO2),
As (up to 8.2 wt. % As2O5) and P (up to 5.9 wt. % P2O5),
slight depletion in U and negligible content of Y+REE
are typical of the most altered zones. The altered zone
appears to be enriched in Nb, Ti, Ta and Th relative to its

Uranium–niobium-rich alteration products of the “písekite” aggregates

1 cm
Fig. 1 The radial aggregate of “písekite” associated with schorl andmonazite-(Ce) in albite. U Nového rybníka quarry, Písek area.

precursors. The EMPA totals of altered areas vary in therange of 88–96 wt. % of oxides (Tab. 1).
Two texturally and compositionally distinct assemblages of secondary U,Nb-rich minerals were recognizedin the “písekite” aggregates. (i) Within the “písekite” aggregate occur small (~ 200 µm ) irregular aggregates ofrandomly oriented flakes of Fe-rich smectite, ~20 µm insize (Fig. 3a). They are interstitial among early replacement products of the “písekite” alteration. Chamosite,which is present in relics in the next assemblage andwas the likely precursor to the Fe-rich smectite, was notfound. (ii) Small pocket in primary albite, about several
mm3 in volume, was observed adjacent to an elongated“písekite” aggregate (Fig. 3b). The pocket is almost
completely filled by a mixture of chamosite and a clay
mineral (Fe-rich member of the smectite group) as randomly oriented flakes, ~ 50–150 µm in size. Chamosite
commonly forms internal parts of flakes and it is compositionally transitional to more common Fe-rich smectite.
Its flakes are locally folded and deformed (Fig. 3b–c).
The pocket is filled, along with dominant chamosite/
smectite, by several other minerals: xenotime-(Y) ~columbite-(Fe) > zircon ~ fluorapatite ~ pharmacosiderite> monazite-(Nd) (Fig. 3b–c). Columbite-(Fe) is evidently
concentrated to the contact with “písekite” pseudomorph
and its elongated euhedral crystals, up to ~150 µm long,
are generally parallel to the contact with “písekite”(Fig. 3b). Xenotime-(Y) forms euhedral grains, ~100 µm
in diameter, commonly located in the centre of the pocket(Fig. 3b). It is closely associated with euhedral crystalsof zircon, up to 30 µm in diameter (Fig. 3b). Textural
relations and morphology of the grains rather indicatethat these minerals probably crystallized as early phasesin the pocket, preceding the chamosite. Fluorapatite andpharmacosiderite are always closely associated as irregular intergrowths, ~50 µm in size (Fig. 3c) concentrated in
the parts of the pocket more distant from the “písekite”
aggregate. Pharmacosiderite obviously replaces fluorapatite grains.

4.2. Secondary U, Nb-rich phases
4.2.1. Secondary assemblage developed
within central parts of the “písekite”
aggregates

0.5 mmFig. 2 Typical BSE image of “písekite” aggregate. Rare relics of intergrown primary polycrase-(Y), samarskite-(Y) and fergusonite-(Y) incentral part are enclosed in weakly to moderately altered counterpartsand surrounded by strongly altered rim.

In this assemblage, volumetrically dominant Fe-richsmectite is intensively replaced by Nb,Si,Ti,Fe,U,Ca-richphase A of highly variable composition (Tab. 1). It is
dominated by Nb2O5 (49.5–57.7 wt. %), SiO2 (6.3–11.8
wt. %), TiO2 (6.4–7.7 wt. %), Fe2O3 (4.1–5.9 wt. %),UO3 (1.3–4.0 wt. %) and CaO (2.3–2.9 wt. %) prevailing over As2O5 (< ~1 wt. %). Further oxides are present
in the concentrations ≤ 1 wt. %; e.g., ~0.3 wt.% ThO2
(Tab. 1), whereas REE were below the detection limit of
the EMPA. The sum of the oxides is c. 98 wt. %. X-ray
elemental maps and chemical analyses revealed strong
zoning of this micaceous aggregate (Fig. 4, 5a). The
border zone along the contact with the highly altered“písekite” aggregate is preferentially enriched in Si
319

320

10.41

0.00

26.91

9.55

As2O5

Nb2O5

Ta2O5

0.83

2.18

2.57

1.54

1.89

3.03

0.73

Sm2O3

Gd2O3

Dy2O3

Er2O3

Yb2O3

CaO

MnO

9.16

94.23

0.62

1.09

1.30

1.11

1.88

1.57

0.74

0.53

97.30

0.39

0.00

0.01

0.65

4.17

3.24

4.38

3.36

1.19

0.89

25.99

0.62

0.40

1.15

0.46

0.00

6.30

42.17

0.00

0.00

1.93

Nr.

1.37
0.13
0.88
98.90

PbO
K2O
Total

6.71

0.07

3.45

0.00

0.00

0.09

0.10

0.18

0.00

0.63

2.14

10.76

12.73

4.63

8.78

23.96

8.18

5.94

0.17

Na2O

FeO

MnO

CaO

MgO

Dy2O3

Gd2O3

Sm2O3

Nd2O3

Y 2O 3

Al2O3

ThO2

UO3

TiO2

SiO2

Ta2O5

Nb2O5

As2O5

P 2O 5WO3altered“písekite”
8

Analyses numbered in the header of the table are plotted in Figs 5a, d and 6.
Note that smectite* is associated with phase A and smectite** with phase B.
nd – not detected, † – average from 3 analyses.

93.97

0.34

Nd2O3

Total

6.09

14.21

Y 2O 3

1.69

1.26

0.33

Sc2O3

0.46

2.39

1.42

ThO2

FeO

9.41

6.88

UO2

PbO

1.60

1.71

0.00

0.01

18.73

SiO2

TiO2

33.45

0.00

0.00

0.00

P2O 5

9.91

0.67

WO3

polycrase-(Y) samarskite-(Y) fergusonite-(Y)

0.73

0.31

0.39

91.03

0.35

0.09

0.35

5.05

0.07

2.85

0.69

0.07

0.03

0.05

0.11

0.30

3.61

0.21

2.05

7.09

7.16

0.56

57.91

1

phase A

Tab. 1 Representative analyses of minerals making up the “písekite” aggregates (wt. %)

82.12

0.86

0.06

0.45

2.71

0.05

2.02

0.21

0.09

0.06

0.09

0.11

0.23

1.89

0.10

22.96

4.11

3.35

0.00

35.65

3.63

0.94

0.55

U-richphase A
2

0.62

8.75

0.21

0.40

0.13

86.10

1.40

0.05

0.08

8.67

0.03

0.84

1.24

0.00

0.00

0.00

0.00

0.00

16.35

0.12

0.53

2.42

41.26

3

0.25

0.93

0.51

0.42

0.14

90.44

0.69

0.05

0.00

10.69

0.03

0.53

0.10

0.00

0.00

0.00

0.00

0.03

28.84

0.00

0.25

0.49

42.49

4

smectite* smectite**

93.00

1.71

0.13

0.49

11.30

0.04

2.73

0.26

0.00

0.00

0.00

0.07

0.07

7.92

0.01

13.06

9.10

19.92

0.07

10.86

3.93

5.63

0.70

95.25

1.64

0.23

1.05

7.14

0.02

4.13

0.09

0.09

0.08

0.06

0.00

0.15

3.66

0.03

22.73

12.45

6.65

0.00

11.00

8.90

8.70

0.45

partial replacement ofsmectite by phase B
5
6

9.59

8.80

0.33

96.58

1.29

0.11

2.22

4.74

0.11

4.70

0.00

0.16

0.10

0.00

0.00

0.28

1.62

0.00

22.90

12.24

1.74

0.00

18.65

7

phase B

6.68

2.96

0.60

1.42
101.47

86.56

nd

nd

0.16

1.23
0.09

4.03

nd

0.25

0.10

nd

nd

nd

nd

nd

0.14

nd

60.81

nd

0.28

nd

nd

17.54

3.25

nd

kahlerite†

5.33

0.04

2.88

0.00

0.30

0.04

0.11

0.05

0.10

0.62

0.12

39.73

4.79

1.14

0.38

23.86

9

phase C

Radek Škoda, Milan Novák, Jaroslav Cícha

Uranium–niobium-rich alteration products of the “písekite” aggregatesaSmeand Al (smectite phase), whereas the central parts are
depleted in Si and Al and enriched in Nb, Ti, Ca and Y.
Uranium shows local enrichments (up to 23.0 wt. % UO3)
in the central parts (Fig. 4, 5a, Tab. 1).

4.2.2. Secondary U, Nb, Ti, Fe, As, P-phasesin the albite pocket

altered“písekite”baltered“písekite”
Ab Col
Zrn

Xtm

Ab

c

Sme
Chl
Psd
Kah
Ap
Ab

Fig. 3 Back-scattered electron (BSE) images of the typical “písekite”related assemblages. a –“Písekite” aggregate; b–c Albite pocket. Mineral names abbreviations: albite (Ab), fluorapatite (Ap), zircon (Zrn),
chamosite (Chl), columbite-(Fe) (Col), kahlerite/metakahlerite (Kah),pharmacosiderite (Psd), smectite-group mineral (Sme), xenotime-(Y)(Xtm). The scale bar is 100 µm in all figures.

The smectite flakes in the pocket are locally intensively
replaced by U,Nb,Ti,Fe,As,P-phases of variable chemical composition. This replacement propagates along thecleavage and is spatially associated with aggregates of“písekite” and crystals of columbite-(Fe) (Fig. 5b–c)
or xenotime-(Y) (Fig. 5d). The replacement, spatiallyassociated to “písekite” and columbite-(Fe), is the mostadvanced along the contact with these phases, generally
decreasing in intensity farther away. The spot (3 µm in
diameter) electron-microprobe analyses differ significantly, showing a continuous variation from smectite to thephase B of the complex composition (Tab. 1): UO3 (22.9
wt. %), Nb2O5 (18.7 wt. %), TiO2 (12.2 wt. %), As2O5 (9.6
wt. %), P2O5 (8.8 wt. %), Fe2O3 (5.2 wt. %) and CaO (4.7
wt. %). The Th and Ta contents were below the detection limit of EMPA. The sum of the oxides is ~ 94 wt.
%. Replacement of Fe-rich smectite spatially associatedto xenotime-(Y) crystals, which are located farther fromthe “písekite” aggregate, produced very similar textures(Fig 5d) but the chemical composition differs, especiallyby higher contents of Nb and U. The most enriched andrather homogeneous phase C contains (see Tab. 1): UO3(39.7 wt. %), Nb2O5 (23.9 wt. %), As2O5 (6.7 wt. %),Fe2O3 (5.8 wt. %), TiO2 (4.8 wt. %). The sum of theoxides is ~99 wt. %.
Further uraniferrous phase found in this pocket forms
aggregates, up to 50 µm in size, with sharp contact tochlorite/smectite. These aggregates frequently enclose
relics of replaced fluorapatite (Fig. 5c) or occur in association with pharmacosiderite–fluorapatite intergrowths(Fig. 5d). The chemical composition of these aggregatesis close to kahlerite or metakahlerite (average of 3 spot
analyses): UO3 (60.8 wt. %), As2O5 (17.5 wt. %), FeO(4.03 wt. %), P2O5 (3.25 wt. %) and CaO (0.3 wt. %).
Other elements such as Mg, Na, Al and K are also present
at levels below 0.25 wt. % of their respective oxides. The
formula recalculation based on 2 cations in the tetrahedral
site yields U/(As + P) close to 2:2, and occupation of theinterlayer position ~0.65 apfu.

5. Discussion
Uranium-rich secondary phases forming at the expenseof complex Nb,Ta,Ti,W-oxide minerals are rather rarein granitic pegmatites (e.g., Novák et al. 2008). At the321Radek Škoda, Milan Novák, Jaroslav Cícha

YL
La


100 µm

BSE

Nb L
La


Ti K
Ka


Ta L
La


Si K
Ka


Al K
Ka


Fe K
Ka


Th M
Ma


UM
Mb


As L
La


Zr L
La


PL
La


Fig. 4 X-ray elemental map of the secondary assemblage within “písekite” aggregate.
Relative concentrations of the given element
are expressed in false colors, whereby the“warm” shades stand for higher content.
The relative concentrations were acquiredin counts per second. Thus, due to the different sensitivity for individual elements, thegiven color need not correspond to the same
concentrations on different elemental maps.

322

Ca K
Ka


Uranium–niobium-rich alteration products of the “písekite” aggregates

Asm

a
Sme

b
Sm

3

Col

1

Phase A
2

Phase B

altered“písekite”

Zrn

Sme

c

Phase B

d

Psd
Xtm

Altered“písekite”

Phase C
Kah
9

Col
Sme
Ap
Ab

Xtm

Phase B
Sme

Ab

Fig. 5 Back-scattered electron (BSE) images of the phases A, B and C from the assemblages within the “písekite” aggregate (a) and from thealbite pocket (b–d). The abbreviations of mineral names as in Fig. 3 except for altered samarskite (Asm) and samarskite-(Y) (Sm). The scale bar
is 100 µm in all figures.

studied locality near Písek, we were able to distinguishthree U, Nb-rich secondary phases in two mineral assemblages (Figs 3–5). All are evidently enriched in U, Nb,
Ti, As, Fe relative to the original “písekite” aggregates
but the compositions vary greatly. The phase A from thefirst assemblage (i), i.e. occurring within the “písekite”
aggregate, shows Nb > Ti > Fe > U > As, whereas thephase B close to the “písekite” aggregate has U ~ Nb >Ti > As > P > Fe and the phase C close to xenotime-(Y)grains exhibits U > Nb > As > Fe > Ti. Both latter phasescome from the albite pocket assemblage (ii). Disregarding the contents of Ta and Th, the chemical compositionof strongly altered “písekite” is very similar to the phase
B, which occurs adjacent to the “písekite” aggregates(Tab. 1, Fig. 6). Hence, “písekite” aggregate was mostprobably the source of Nb, Ti and U, whereas thechlorite/smectite minerals provided Fe and Si. High As
contents in some secondary minerals (pharmacosiderite,
kahlerite/metakahlerite) and in altered zones of “písekite”

aggregate together with the absence of As in primaryminerals (samarskite-(Y), polycrase-(Y), monazite-(Ce,Nd), fluorapatite) imply a participation of late fluids
released during the arsenopyrite decomposition. It is acommon accessory mineral in the Obrázek I pegmatite
but occurs far from the “písekite” aggregates. In such
likely acidic environment, apatite becomes unstable (e.g.,Morton and Hallsworth 1999) and its dissolution may
have released P present in alteration products but absent
in the “písekite” aggregate itself (Tab. 1). However, altered feldspars also may be a source of P (e.g., London
et al. 1990). Pharmacosiderite, a typical low-T alteration
product of arsenopyrite (Drahota and Filippi 2009) as
well as a smectite mineral (Dekov et al. 2008) provide
an evidence that the hydrothermal alteration process was
low-T (< ~150–200 °C).
Given the limits of the analytical technique used inthis study (EMPA), we cannot explicitly decide whether
the examined phases A, B and C are individual miner323

Radek Škoda, Milan Novák, Jaroslav Cícha
100
Fe2O3

80

wt.% oxides

CaO
Al2O3
ThO2
UO3
TiO2
SiO2
Ta2O5
Nb2O5
As2O5
P2O5

60

40
smectite

20

0
4

5

6

7

8

altered“písekite”
phase B

Col

Kah
8
7

5

6

4

Ab
Sme
4.0
3.0

Fe
2.0

apfu

Nb
Ti
Ta

P
As
U
Ca

Si
Al

1.0

0.5

0.0
4

5

6

7

8

Fig. 6 Chemical composition of smectite, phase B and their transitional
phases expressed in wt.% oxides (upper plot) and atoms per formula
unit (apfu) in the lower plot. The mineral formulae were calculated bynormalization to 12 anions, whereas Fe was considered as exclusively
trivalent. The locations of analysed spots 4–8 are marked in the BSE
image (center); the scale bar is 100 µm. The analyses are presented in
Tab. 1; 4 – Fe-rich smectite, 5–6 – transitional phases, 7 – phase A,
8 – altered “písekite”.

als or mixtures of several phases. However, their quite
complex chemical compositions suggest rather a mixture of nanoscale, possibly amorphous hydrated oxides,
phosphates–arsenates and silicates of U, Fe and Ca. In
addition, the recalculation of the mineral formula on thebasis of oxygen atoms did not provide any reasonable
elemental ratios. Transitional chemical composition be324

tween these phases and Fe-rich smectite (Tab. 1, Fig. 6)
probably represents only partial replacement of smectite
by newly-formed phases in analytical volume of EMPA
(few µm3). Despite the fact that they occur in various
parageneses, the A, B and C phases replaced exclusivelya smectite mineral. Hence smectite seems an appropriate
precursor for low-T replacement at acidic conditions.
Kahlerite/metakahlerite is the only secondary U-richphase that is close in composition to a recognized mineral. It differs from the other U,Nb-rich phases by theabsence of Nb and Ti and, moreover, the textural andtextural relations (Figs 3c, 4c), which indicate that it
replaced early-formed fluorapatite and pharmacosiderite.
The contacts with chlorite/smectite minerals are sharpwith no indication of replacement. Hence, its origin
seems different from the phases A, B and C, which evidently replaced chlorite/smectite aggregates.
The alteration of metamict “písekite” aggregates (polycrase-(Y) and samarskite-(Y) in the examined samples)
led to a loss of Y + REE, and concomitant increase in Ca,
Si, As, P and water contents in newly-formed secondary
minerals (Tab. 1, Fig. 6). Release of Y + REE and theirfurther absence in secondary phases suggests very highmobility of such elements at the given conditions. The
chemical analyses also indicate that U and Th have atendency to be incorporated into the altered (hydrated, Si,
As, P, Ca, Fe-rich) parts of “písekite” aggregate. The additional dissolution of these altered parts led to a releaseof U, which precipitated in a form of iron uranyl arsenate(kahlerite/metakahlerite) or secondary U, Nb-rich phases
A, B and C. Absence of secondary Th-bearing phases,very low Th contents in A, B, C phases, and decreasing
U/(U + Th) ratio from the least to the most altered partsof “písekite” aggregate indicate evidently lower mobility of Th than U, probably due to the oxidation of U 4+ tomore mobile U6+.
The elucidation of alteration mechanisms producing
U, Nb-rich phases is complicated by the very probableamorphous nature of the newly-formed phases and theirsmall size. However, laboratory experiments and textural
relations indicate that the dissolution and precipitation
model (Pöml et al. 2007) operated. Similar compositionalchanges such as hydration, release of Y + REE and increase in Ca and Si during the alteration of Y, REE, Nb,
Ta, Ti-oxide minerals were recorded by several authors(e.g. Nasraoui and Bilal 2000; Ercit 2005; Geisler et al.
2005; Ruschel et al. 2010).

6. Conclusions“Písekite” aggregate, a fine-grained mixture of mostly
metamict U, Th-enriched Y, REE, Nb, Ta, Ti-oxide minerals (primary precursors – polycrase-(Y) ~ samarskite-Uranium–niobium-rich alteration products of the “písekite” aggregates(Y) > fergusonite-(Y)) from Obrázek I pegmatite, Písek,underwent a low-temperature hydrothermal alteration at< 150–200 °C. The secondary assemblages are dominatedby Fe-rich smectite and include also pharmacosiderite and kahlerite/metakahlerite. The altered zones of“písekite” aggregate are depleted in Y and REE > U andsignificantly enriched in Si, As and P. The newly-formed
products include U, Nb, Ti, Fe, As, P-rich phases of variable chemical composition: phase A (Nb > Ti > Fe > U> As), phase B (U ~ Nb >Ti > As > P > Fe) and phase C(U > Nb > As > Fe > Ti). The textural features show that
replacement of the Fe-rich smectite by the U, Nb-richphases took place on a micrometer scale. The highly heterogeneous U, Nb-rich phases likely represent a mixtureof nanoscale hydrated oxides, phosphates–arsenates aswell as silicates of U, Fe and Ca.
Acknowledgements. The authors thank to the reviewers
A. Deditius and P. Uher, and the Guest Editor J. Plášil forthe constructive reviews, which significantly improved
the original manuscript. This work was supported by the
project of Grant Agency of Academy of Czech Republic no. KJB 301630801 to RŠ and the research project
MSM0021622412 (INCHEMBIOL) to MN.

References
Bouška V, Johan Z (1972) New data on písekite. Lithos
5: 93–103
Breiter K, Cempírek J, Kadlec T, Novák M, Škoda R(2010) Granitic pegmatites and mineralogical museumsin Czech Republic. Acta Mineral Petrogr, Field Guide
Series 6: 1–56
Čech F (1985) Mineralogy of Granitic Pegmatites of
the Czech Part of the Bohemian Massif. Unpublisheddissertation, Charles University, Prague, pp 1–333 (inCzech)
Černý P, Ercit T S (2005) The classification of granitic
pegmatites revisited. Canad Mineral 43: 2005–2026Černý P, Novák M, Chapman R, Ferreira KJ (2007)
Subsolidus behavior of niobian rutile from the Písek region, Czech Republic: a model for exsolution in W- andFe2+>>Fe3+-rich phases. J Geosci 52: 143–159
Dekov VM, Caudros J, Shanks WC, Koski RA (2008) Deposition of talc – kerolite–smectite – smectite at seafloorhydrothermal vent fields: evidence from mineralogical,
geochemical and oxygen isotope studies. Chem Geol
247: 171–194
Drahota P, Filippi M (2009) Secondary arsenic minerals in the environment: a review. Environ Internat 35:
1243–1255
Ercit TS (2005) Identification and alteration trends ofgranitic-pegmatite-hosted (Y,REE,U,Th)–(Nb,Ta,Ti)oxide minerals: a statistical approach. Canad Mineral
43: 1291–1303
Ewing RC (1975) Alteration of metamict, rare-earth, AB2O6type Nb–Ta–Ti oxides. Geochim Cosmochim Acta 39:
521–530
G eisler T, Seydoux-G uillaume AM, Pöml P, G ollaSchindler U, Berndt JR, Wirth R, Pollok K, Janssen A,
Putnis A (2005) Experimental hydrothermal alteration of
crystalline and radiation-damaged pyrochlore. J Nuclear
Mater 344: 17–23
Krejčí A (1923) Písekite, a new radioactive mineral or
pseudomorph. Čas Mineral Geol 1: 1–5
London D, Černý P, Loomis JL, Pan JJ (1990) Phosphorus
in alkali feldspars of rare-element granitic pegmatites.
Canad Mineral 28: 771–786
Morton A, Hallsworth CR (1999) Processes controllingthe composition of heavy mineral assemblages in sandstones. Sed Geol 124: 3–29
Nasraoui M, Bilal E (2000) Pyrochlores from the Lueshe
carbonatite complex (Democratic Republic of Congo): ageochemical record of different alteration stages. J Asian
Earth Sci 18: 237–251
Novák M, Černý P, Kimbrough DL, Taylor MC, Ercit TS(1998) U–Pb ages of monazite from granitic pegmatites
in the Moldanubian Zone and their geological implications. Acta Univ Carol, Geol 42: 309–310
Novák M, Johan Z, Škoda R., Černý P, Šrein V, Veselovský F (2008) Primary oxide minerals in the system
WO3–Nb2O5–TiO2–Fe2O3–FeO and their breakdown
products from the pegmatite No. 3 at Dolní Bory-Hatě,Czech Republic. Eur J Mineral 20: 487–499
Pöml P, Menneken M, Stephan T, Niedermeier DDR,
Geisler T, Putnis A (2007) Mechanism of hydrothermal
alteration of natural self-irradiated and synthetic crystalline titanate-based pyrochlore. Geochim Cosmochim
Acta 71: 3311–3322
Pouchou, JL, Pichoir, F, (1985) “PAP” (φρZ) procedure for
improved quantitative microanalysis. In: Armstrong JT(ed) Microbeam Analysis. San Francisco Press, pp 104–106
Povondra P (1981) The crystal chemistry of tourmalinesof the schorl–dravite series. Acta Univ Carol Geol 1981:
223–264
Ruschel K, Nasdala L, Rhede D, Wirth R, Lengauer Ch,
Libowitzky E (2010) Chemical alteration patterns inmetamict fergusonite. Eur J Mineral 22: 425–433
Sejkora J, Litochleb J, Exnar P, Černý P, Čech F (1998)
Some physico-chemical data for pink beryl (morganite) from the Písek pegmatites. Věst Čes Geol Úst 73:
347–350 (in Czech with English summary)Škoda R, Novák M, Gadas P, Cícha J, Heroldová N (2010)
Hydrothermal alteration of Y, REE, Nb, Ta, Ti-oxideminerals from the Písek pegmatites, Czech Republic:a low-temperature As enrichment. Acta Mineral Petrogr,
Abstract Series 6: 475
325